I've always wanted a "modern" Amiga laptop so how about a motherboard for the upcoming Olimex TERES-I Do-It-Yourself Open Source Hardware and Software Laptop https://www.olimex.com/Products/DIY-Laptop/. I'm not sure if the open source nature of the project would be an issue for the closed source Apollo Core but I don't see why it should be as processor implementations are rarely open source so just because the core is currently on an FPGA rather than an ASIC why shouldn't it be allowed, BigGun even recently said he had no objection to SAGA being open sourced. The other issues are the keyboard could be less Linux specific and it could be bigger, the display is only 11.6" so it's closer to a netbook/chromebook than most modern laptops.

How about a motherboard for a "retro" games console. I'm thinking of a "VampireCDStation32", take a Playstation one remove the motherboard and replace it with a vampire board. The casing, PSU, drive mechanism, controllers connector unit and controllers of the original unit would all be reused. An HDMI output replaces the AV connector on the back and a micro SD connector (or maybe USB) replaces the Serial I/O connector. Issues include interfacing with the CD mechanism both in hardware and software, supporting controllers (kipper2k already sells a PSX controller to Amiga adaptor with CD32 support so the protocol shouldn't be a problem) but what about PSX mouse support and we may need that USB port or something else on the back if we want keyboard support.

Replacement motherboards for existing Amiga case(s). The obvious one to start with is the A1200 as new cases are now possible. If cooperation and funding could be found a new lower case molding with sockets openings layout and labels appropriate to a new Vampire board (HDMI, Joysicks, ethernet, barrel jack PSU, rear USB, CF Card and (micro) SD Card, 3.5mm stereo audio headphone jack, mic jack) and maybe a slim optical drive bay on the back.

Tablets and/or phones. Issues include support for touch control beyond mouse/pointer control with the existing operating systems and all the other software/firmware required, obtaining parts without buying an off the shelf tablet/phone and swapping out the main board (RF chips/parts for a phone may be and issue), challenge of designing and building using tiny part and squeezing all the components in for a phone, lack of any support from common mobile apps (performance may not be enough for an Android compatibility layer and as Blackberry and Microsoft found, without full apps support a mobile platform is doomed).